Cellulosic Films Obtained from the Treatment of Sugarcane Bagasse Fibers with N-methylmorpholine-N-oxide (NMMO)

Cellulosic Films Obtained from the Treatmentof Sugarcane Bagasse Fiberswith N-methylmorpholine-N-oxide (NMMO)

Denise S. Ruzene & Daniel P. Silva & António A. Vicente &

José. A. Teixeira & Maria T. Pessoa de Amorim &

Adilson R. Gonçalves

Received: 22 May 2008 /Accepted: 16 January 2009 /Published online: 13 February 2009# Humana Press 2009

Abstract Ethanol/water organosolv pulping was used to obtain sugarcane bagasse pulpthat was bleached with sodium chlorite. This bleached pulp was used to obtain cellulosicfilms that were further evaluated by Fourier transform infrared (FTIR) spectroscopy,thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). A good filmformation was observed when temperature of 74 °C and baths of distilled water were used,which after FTIR, TGA, and SEM analysis indicated no significant difference between thereaction times. The results showed this to be an interesting and promising process,combining the prerequisites for a more efficient utilization of agro-industrial residues.

Keywords Agro-industrial residue . Sugarcane bagasse . Cellulose fiber . Cellulose film .

NMMO

Introduction

Due to the growing emphasis on decreasing pollutant emissions, there has been significantscientific and technological interest in the development of nonpolluting processes based onthe use of organic solvent of cellulose [1].

Sugarcane bagasse, a residue obtained after the manufacture of sugar and ethanol, is themost abundant lignocellulosic residue in Brazil. According to Sun et al. [2], in general,

Appl Biochem Biotechnol (2009) 154:217–226DOI 10.1007/s12010-009-8529-8

D. S. Ruzene (*) : A. R. GonçalvesDepartment of Biotechnology, Engineering School of Lorena, University of São Paulo, P.O. Box 116,12602-810 Lorena, São Paulo, Brazile-mail: [email protected]

D. S. Ruzene :D. P. Silva : A. A. Vicente : J. A. TeixeiraIBB—Institute for Biotechnology and Bioengineering, Center of Biological Engineering,University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal

M. T. Pessoa de AmorimDepartment of Textile Engineering, Center of Science and Textile Technology, University of Minho,4800-058 Guimarães, Portugal

1 ton of sugarcane generates 280 kg of bagasse; thus, with one production of about 455×106 ton of sugarcane, Brazil produced 127×106 ton of bagasse during the year 2006 [3].Although most of the bagasse has been employed in the sugarcane industry to generateenergy, there is a surplus of this agro-industrial residue, and several alternatives for itsutilization have been evaluated, among which the production of animal feed, ethanol, paper,enzymes, and food additives, without impacts on the food and feed production or evendamaging effects on native forests. Sugarcane bagasse is constituted by three maincomponents: cellulose, lignin, and hemicelluloses [4].

Organosolv processes, pulping procedures utilizing aqueous organic solvents, havebeen extensively studied in the last 30 years as an alternative to chemical processes ofpulping [5–9]. These processes can collaborate largely to the decrease of theenvironmental impact caused by conventional delignification processes, besides allowingfor the integral use of lignocellulosic components in chemical products of commercialinterest [6, 10, 11]. The ethanol/water process combines high efficiency, low cost, andethanol abundance, an advantage in countries where sugar cane is economically important[6, 7, 12].

Cellulose is a linear and high-molecular-weight polymer as well as natural, renewable,and biodegradable material [13]. Because of its intermolecular and intramolecular hydrogenbonds, cellulose is not dissolved by common solvents [14] but is dissolved by systemswhich include heavy metal–amine complex solutions, concentrated metal salts, cold NaOHsolutions, thiocyanate/amine, LiCl/dimethylacetamide, N-methylmorpholine-N-oxide(NMMO)/H2O system, and concentrated H2SO4 and H3PO4 [15, 16].

The cellulose fibers produced by direct dissolution of cellulose in NMMO have the genericname of lyocell [17, 18]. This process is environmentally benign because the nontoxicNMMO solvent is used and almost all the solvent used is totally recycled [19]. Lyocell fiberproduction should be an entirely physical process that does not cause chemical changes inthe pulp or solvent. However, there are several side reactions and considerable by-productformation in the system cellulose/NMMO/water which can cause harmful effects, such asdegradation of cellulose, pronounced decomposition of NMMO, and decreased productperformance. Very few chemicals are applied, and, in the idealized case, NMMO and waterare completely recycled, which is also an important economic factor [18].

The dissolution of cellulose in NMMO/water mixtures is a physical process that does notrequire chemical derivatization of the solute [18, 20]. However, the processes occurring atthe molecular level during dissolution of cellulose in NMMO remain largely unknown.Swelling and dissolution of cellulose must be caused by rearrangement of the hydrogenbond networks in the system, meaning breakage of intramolecular and intermolecular Hbonds in cellulose and in NMMO/water, with concomitant formation of new H bondsbetween solvent and cellulose [16, 20].

The present study describes the application of NMMO in sugarcane bagasse pulp thatwas obtained by ethanol/water organosolv process under basic conditions and bleachedwith sodium chlorite. This bleached pulp was used to obtain cellulosic films that werestudied by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis(TGA), and scanning electron microscopy (SEM). FTIR spectroscopy has been largely usedfor the characterization of lignocellulosic materials and studied in some works involvingcharacterization of the compounds with or without modification [21]. SEM is one of themost versatile techniques available for the analysis of the microstructural characteristics ofsolid objects and permits the observation and characterization of heterogeneous organic andinorganic materials on nanometer (nm) to micrometer (μm).

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Materials and Methods

Ethanol Pulping Procedure

Pulping of sugarcane bagasse with ethanol/water 1:1 (v/v) mixture was carried out in aclosed and pressurized vessel using NaOH (5% dry bagasse) at 185 °C for 3 h. Theproducts were filtered and the pulp was dried for determination of yield.

Pulp Bleaching Procedure

Dried and refined pulps were suspended in water (3% consistency) and heated to 70±5 °C.Sodium chlorite and glacial acetic acid were added 2.5:1 (w/v). The solution was furtherheated to 70±5 °C for 60 min. The samples were cooled in ice bath to 10 °C. Bleached pulpwas filtered, washed with distilled water, and dried [22].

Analyses and Chemical Composition of the Pulp

Kappa number and viscosity of the pulp were determined by standard methods [23,24]. One gram of dry pulp was treated with 10 mL of 72% H2SO4 under stirring at 45 °Cfor 7 min. The flask was autoclaved for 30 min at 1.05 bar for the complete hydrolysis ofoligomers. After filtration through a Sep-Pak C18 cartridge to remove aromaticcompounds, the hydrolysate was analyzed in an Aminex HPX-87 H column (300×7.8 mm2, Bio-Rad) at 45 °C using a Shimadzu chromatograph with refractive-indexdetector. The mobile phase was 0.005 mol/L H2SO4 at 0.6-mL/min flow rate. Sugarconcentrations, reported as glucan and xylan, were determined from calibration curvesobtained with pure compounds. Lignin was determined by gravimetric analysis [25]. Allexperiments were performed in triplicate.

Determination of Brightness

The brightness of pulps was determined in agreement with the Technical Association of thePulp and Paper Industry (TAPPI) [26]. Samples were prepared following the TAPPI norm[27]. After 24 h, the sheets with thickness between 310 and 315 g/m2 were analyzed usingPhotovolt 577 equipment. The reflection percentage was determined at five different pointsand the results were presented as average values.

Film Preparation

Samples of 0.5 g of bleaching pulp were transferred to 100-mL flasks and put in awater bath at 74 °C. Then, 2.5 g H2O and 13.7 g NMMO were added. The filmpreparation was started when the temperature reached 74 °C and nitrogen was added, for1.5 and 2.5 h. After the indicated period of reaction, samples were removed with a widespatula and put on a Teflon plate (9×5 cm) and cast as a film (3×1 cm) that was washed intwo consecutive baths of distilled water. Films were dried at room temperature for 24 h andput under vacuum to dry at 35 °C for 4 h. Other two samples were heated at 50 °C (afterdissolving at 74 °C) and were cooled down to room temperature without the aid of thedistilled water baths. The NMMO used was 4-methylmorpholine-4-oxide monohydrate(Sigma).

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Fourier Transform Infrared of Samples

FTIR spectra of samples were obtained on a Fourier transform infrared spectrophotometer(ABB FTLA 2000) operating at 4-cm−1 resolutions and using a KBr disc containing 1%finely ground samples.

Thermogravimetric Analyzer

The thermal stability of the samples was evaluated using TGA on a thermal analyzer (TGA-50 Shimadzu, Japan). The apparatus was continually flushed with nitrogen. The samplesweighed between 10 and 12 mg and were run from room temperature to 580 °C at a rate of10 °C/min.

Scanning Electron Microscopy

The samples of sugarcane bagasse and unbleached and bleached pulp were mounted onstubs, freeze-dried, coated with gold, examined, and photographed in a scanning electronmicroscope SEM/EDS:JEOL JSM 6301F/Oxford INCA Energy 350. The micrographs ofNMMO and films were obtained by scanning electron microscopy in low vacuum using amicroscope FEI Quanta 400FEG.

Results and Discussion

The composition results based on dried weight of the sugarcane bagasse, original pulp(unbleached), and bleached pulps are listed in Table 1. Sugarcane bagasse material wascharacterized by its high lignin content (28% w/w), which was two and four times higherthan that of the unbleached and bleached pulps, respectively. However, ethanol/waterpretreatment of original (unbleached pulp) and bleached pulps from sugarcane bagasseremoved a significant amount of lignin (48% and 76%, respectively). The kappa number ofthe unbleached pulp was high (≅ 46), indicating that the lignin present in the pulp was notremoved after the pulping process. On the other hand, bleached pulp presented a low kappanumber (micro-kappa number of 2.4). Viscosity of bleached pulp was lightly decreasedafter bleaching (from 8.1 to 7.8 cPa), indicating that the average cellulose chain length(polymerization degree) was less reduced [21]. The brightness of bleached pulps increased2.5 times after bleaching using chlorite and 69.5% is a good value for brightness

Table 1 Chemical composition of sugarcane bagasse, unbleached pulp, and bleached pulp.

Component Composition

Sugarcane bagasse Unbleached pulp Bleached pulp

Glucan (%) 43.7±0.9 55.7±0.6 64.6±0.5Hemicellulose (%) 23.7±0.5 23.2±0.5 21.9±0.5Total lignin (%) 28.1±1.9 14.6±0.9 6.6±0.5Kappa number – 46.2±0.6 2.4a±0.2Viscosity (cPa) – 8.1±0.2 7.8±0.3Brightness (%) – 27.1±0.8 69.5±1.6

aMicro-kappa number

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considering both pulping and bleaching processes. Shatalov and Pereira [28] achieved76.4% brightness in an organosolv pulp from Arundo donax after hydrogen peroxidebleaching.

Subsequently, the bleached ethanol/water pulp obtained in this work was used to obtaincellulosic films with NMMO. The present work presents the results of two differenttreatments of film: (1) at 74 °C for 1.5 and 2.5 h, with subsequent wash in distilled water(films 1 and 2, respectively); and (2) at 50 °C for 1.5 and 2.5 h and cooled down to roomtemperature without bath of distilled water (films 3 and 4, respectively). We chose twomethods of film preparation because it was observed that when we used water bath thefilm did not crystallized and when we did not use water bath crystallized. We saw thesimple formation of the film with water bath and without water bath crystallizationoccurring. The film obtained after treatment at 50 °C for 2.5 h crystallized, while for onecarried out at 1.5 h the pulp was not dissolved and crystallized. We used 74 °C because itis the dissolution temperature of NMMO and we used 50 °C to compare the differencesbetween these temperatures in relation to the second subject. The bleached pulp and thefilms obtained were analyzed by FTIR, TGA, and SEM and the results are shown inFigs. 1, 2, and 3.

Fourier Transform Infrared

Figure 1 shows the FTIR spectrum from unbleached and bleached pulps, from filmsobtained by NMMO, and from the NMMO reagent itself. This figure shows only thespectra between 800 and 2,000 cm−1 because no changes are observed in the interval of2,000–4,000 and 600–800 cm−1. According to Zhao et al. [29], strong H bonds formbetween cellulosic groups and the N–O group of the NMMO but no significant peak shiftswere observed when cellulose and NMMO mixture samples are heated at differenttemperatures. The same behavior was observed in the present work.

New peaks at 1,575 and 1,632 cm−1 were observed when pulp and NMMO were used.The peak at 1,632 cm−1 was assigned to H2O bonded to cellulose [30]. The peak at1,575 cm−1 is very close to water vapor at 1,595 cm−1 [29]. The peak at 1,600 cm−1 must beattributed to H2O molecules with almost no hydrogen bonding; it is known that cellulosemolecules compete with H2O molecules that are bonded to NMMO. This means that some

2000 1800 1600 1400 1200 1000 8000.0

0.5

1.0

1.5

2.0

2.5

1575

1632

7

6

5

4

3

21

Abs

orba

nce

Wavenumber (cm-1)

1100Fig. 1 FTIR spectra of un-bleached pulp (line 1), bleachedpulp (line 2), NMMO (line 3),film 1 with 1.5 h of treatment(line 4), film 2 with 2.5 h oftreatment (line 5), film 3 with1.5 h of treatment without bath(line 6), and film 4 with 2.5 h oftreatment without bath (line 7)

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H2O molecules may be replaced by cellulose molecules and the other part of released H2Omolecules is physically confined in the cellulose matrix and forms no or very weakhydrogen bonds with cellulose. The peak at 1,100 cm−1 is typical for ether bonds and notmuch specific for different sources. But it is seen that in films this band is higher in

(A)

(C)

(B)

Fig. 3 Scanning electron microscopy of sugarcane bagasse (a), unbleached pulp (b), and bleached pulp (c)

12

3

4

6

5

7

8

Fig. 2 TGA curves of sugarcane bagasse (line 1), unbleached pulp (line 2), bleached pulp (line 3), NMMO(line 4), film 1 with 1.5 h of treatment (line 5), film 2 with 2.5 h of treatment (line 6), film 3 with 1.5 h oftreatment without bath (line 7), and film 4 with 2.5 h of treatment without bath (line 8)

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comparison with the band in NMMO or in pulps, giving indication of an etherification thatcan occur in the film prepared.

Thermogravimetric Analysis

TGA is an analytical technique used to determine a material’s thermal stability and itsfraction of volatile components by monitoring the weight change that occurs when a sampleis heated. Synthetic and natural polymers are subject to a degradation of the mechanicalproperties under the influence of increased temperatures [31]. In this work, the thermalproperties (Fig. 2) of sugarcane bagasse (line 1), unbleached pulp (line 2), bleached pulp(line 3), NMMO (line 4), film 1 with 1.5 h of treatment (line 5), film 2 with 2.5 h oftreatment (line 6), film 3 with 1.5 h of treatment without bath (line 7), and film 4 with 2.5 hof treatment without bath (line 8) were studied by TGA.

Figure 2 and Table 2 show the TGA results obtained from sugarcane bagasse fibers andfilms obtained by using NMMO. In general, there are three stages of degradation in theTGA curves of natural fiber samples. The initial weight loss of fibers (100–150 °C) is dueto evaporation of the adsorbed moisture. This loss depends on the initial moisture content ofthe fibers. The second severe weight loss (250–350 °C) and the third stage are due todecomposition of the major components of the fibers. These results are consistent with thedata reported in the literature [32, 33]. The higher onset of degradation temperaturesindicates the improvement of the material’s thermal stability. These results clearly illustratethat the thermal stability of the sugarcane bagasse fibers increases after chemical treatment.The lower thermal stability of the untreated fibers relative to chemically treated fibers isattributed to the higher lignin content in the raw material [33, 34].

The degradation temperature of the NMMO and also of the films reached 160 °C. Thedecrease in the onset value of NMMO and films can be attributed to the decrease in thecrystallinity of the fibers after chemical treatment using NMMO and also means a decreaseof the stability of the mixture (bleached pulp and NMMO). There is also a little distinctionbetween the amounts of the residues remaining after heating of 560 °C heating. All sampleshave a residual weight between 0.35% and 5.9% at 560 °C (Table 2).

Under an inert atmosphere, the final products from degradation of cellulose arecarbonaceous residues plus intact fibers when they do not remain after heating [35]. Duringthe plant growth, inorganic compounds are needed as nutrients, and these inorganiccompounds will generate ash. Samples of sugarcane bagasse and pulps were found to beinitially degraded at about 260 °C whereas NMMO and films obtained by NMMO wouldbe degraded at about 160 °C. Maximum rates of weight loss were observed between 260

Table 2 Degradation characteristics of the sugarcane bagasse fibers, NMMO, and films obtained byNMMO.

Samples Onset of degradation (°C) Residue after 560 °C (%)

Sugarcane bagasse 320 5.9Unbleached pulp 339 5.2Bleached pulp 337 8.1NMMO 217 0.3Film 1 187 2.6Film 2 183 6.1Film 3 182 4.8Film 4 187 3.5

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and 360 °C for sugarcane bagasse and pulps and between 160 and 240 °C for NMMO andfilms. As observed, sugarcane bagasse and pulps were stable up to 250 °C (lines 1–3),whereas NMMO and films were stable up to 150 °C (lines 4–8). Beyond thesetemperatures, thermal degradation takes place. As can be seen (Fig. 2), for a 30% weight

(C) (D)

(E)

(A) (B)

Fig. 4 Scanning electron microscopy of film 1 with 1.5 h of treatment (a), film 2 with 2.5 h of treatment (b)at 74 °C, film 3 with 1.5 h of treatment without bath (c), film 4 with 2.5 h of treatment without bath (d) at50 °C and NMMO (e)

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loss, the decomposition temperatures of the degraded samples occurred at 175 °C (lines 7–8),186 °C (line 4), 190 °C (line 5), 195 °C (line 6), 320 °C (line 1), and 340 °C (lines 2–3).

Scanning Electron Microscopy

The influence of the pulping process as well as of the bleaching process in the structure ofthe sugarcane bagasse particle can be observed in Fig. 3. In this figure, the sugarcanebagasse in the original form (Fig. 3a) and the unbleached pulp (Fig. 3b) shows that thepulping promoted the rupture and solubility of the lignin to the liquor that generated acellulose-rich solid residue. The lignin removal, after bleaching, permitted the separation ofthe cellulose fibers, as can be visualized in Fig. 3c. Surface and cross-sectionalmorphologies of cellulose films were investigated under different conditions.

Figure 4 shows the surface morphologies of cellulose films taken by SEM. When cellulosefilms were washed in water bath, a good formation of film without rough surfaces andhomogeneous structures was observed as shown in Fig. 4a (1.5 h and 74 °C) and Fig. 4b(2.5 h and 74 °C), while films that were not washed in water bath show a surface roughnesswith a layered structure Fig. 4c (1.5 h and 50 °C) and with fibers of bleached pulp stillundissolved Fig. 4d (2.5 h and 50 °C). In this case (Fig. 4c), it is possible to see some fibresin the film; this could be due to the temperature being too low that did not dissolve the pulp at50 °C.

Conclusions

A good formation of film was observed without rough surfaces and originatinghomogeneous structures. TGA analyses show that the decrease in the onset value ofNMMO and films can be attributed to the decrease in the crystallinity of the fibers afterchemical treatment using NMMO; this means a decreased stability of the mixture (bleachedpulp and NMMO). FTIR results suggest that the released H2O molecules exist bothadsorbed and physically confined in the cellulose matrix. The techniques studied furnishinformation on the properties of the material and, when possible, making correspondencebetween themselves. Thus, the process presented in this work showed to be interesting andpromising, combining the prerequisites for a more efficient utilization of agro-industrialresidues.

Acknowledgements The authors acknowledge the financial support from Fundação de Amparo à Pesquisado Estado de São Paulo (FAPESP), Conselho Nacional de Desenvolvimento Científico e Tecnológico(CNPq), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil, as well as theFundação para a Ciência e a Tecnologia (FCT) and CEMUP financial support refa REEQ/1062/CTM/2005and REDE/1512/RME/2005—Fundação para a Ciência e Tecnologia (FCT), Portugal.

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